As a warrior archetype, I leveled smithing. I could damn near always buy everything I needed from vendors in towns (Whiterun, Markarth, Solitude) or easily find it in veins throughout the world. Now as I am playing my Stealth Bow/Dagger archetype, I also choose to level smithing. I just hit around level 18 and finally perked into Elven armor. Not really a problem to find moonstone ingots and ores from vendors, its seems just as common as Dwarven and Orichalum (sp?) ingots and ore from vendors, but not one vendor I've seen has had any quicksilver, ore or ingots. Ive been to three towns (Whiterun, Riften, and Markarth) and nobody is selling any. I haven't been able to find any veins either. Anybody know a good general area to find quicksilver (since the veins/mines seem to be condensed to a general part of the map) or any vendors who sell it. Thank you!

Edit: TL;DR: Where is the best place to find some quicksilver?